1. Field of the Invention
The present invention relates to a flat panel display and a method for driving the same, and more particularly, to a flat panel display for preventing a thin film transistor from deteriorating due to voltage, static electricity, and external force, accidentally applied to a substrate, and a method for driving the same.
2. Discussion of Related Art
Generally, in order to implement a thin film transistor on a conductive thin film formed by using a conductive substrate composed of stainless steel (SUS) or titanium (Ti), a buffer layer is needed between the conductive thin film and the thin film transistor to function as an insulating layer. As such, the thin film transistor formed on the conductive thin film including the buffer layer has electrical and structural characteristics similar to silicon on insulator (SOI) transistor. In the thin film transistor having the buffer layer, a semiconductor layer that forms the thin film transistor can be made of low temperature polysilicon (LTPS), in which an amorphous silicon layer is heated and crystallized at low temperature or is crystallized by excimer laser annealing (ELA).
However, in the thin film transistors as described above, when an external force is applied to the conductive substrate, the thin film transistor including the semiconductor layer formed by the low temperature polysilicon may be deteriorated. In more detail, when a tensile stress is applied to the conductive substrate, the drain current of the thin film transistor is increased, and when compressive stress is applied to the conductive substrate, the drain current of the thin film transistor is decreased.
As such, due to the tensile stress or compressive stress generated when bending the conductive substrate, the drain current of the thin film transistor varies so that characteristics of a flat panel display using the thin film transistor, leakage current and mobility, and brightness and contrast of image pixels are also changed. Thus, an image of the flat panel display is deteriorated.
Moreover, in the structure of the conventional thin film transistor, since when an expected voltage is applied to the conductive substrate and/or unexpected electric charges are accumulated on the conductive substrate due to static electricity, the threshold voltage of the thin film transistor becomes different from a reference threshold voltage, thereby making circuit tuning difficult.
FIG. 1 illustrates changes in characteristic of drain current of a conventional thin film transistor due to application of back bias.
As shown in FIG. 1, the X-axis represents gatebias applied to the thin film transistor, and the Y-axis represents change of the drain current due to the back bias applied to the thin film transistor. As illustrated in this graph, it can be derived that when the back bias voltage is increased, a shift of threshold voltage is increased.
As such, when the threshold voltage of a thin film transistor substrate is changed into a value different from a reference threshold voltage, color coordinates representing every color displayed on a display using the thin film transistor with X- and Y-values, gray scale, and contrast may be changed.